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Composting imported organic-waste residues before applying them to soils may help fight crop diseases. Good composts are costly to buy and slow to produce, but they can pay their own way - especially on farms that produce high-value vegetables and small-berry fruits. At Ohio State University, plant pathologist Harry Hoitink and his co-workers have found that compost may suppress root and foliar diseases. Among the possible reasons:

compost-treated plants are usually healthier and better able to resist infection, compost feeds microorganisms, which produce plant growth hormones and chelates that make micronutrients more available to plants, andcompost hosts beneficial organisms that feed directly on disease organisms, compete with them for nutrients or produce antibiotics.

Some soils or potting mixes blended with medium- maturity compost - which still contains enough food for microorganisms - have sparked systemic resistance in plants, Hoitink says. 'These plants have elevated levels of biochemical activity relative to disease control and are better prepared to defend themselves against diseases.'

Not all composts provide this beneficial effect. In fact, composts and other biological materials that are rich in available nitrogen may actually stimulate some plant diseases. Among these diseases are Phytophthora root rot in soybeans, Fusarium wilts in vegetable crops and fire blight in fruit crops. To reduce the risk of initiating disease, spread these materials many months before cropping, allow the salts to leach away, or blend in low-nitrogen materials before application.

In Ohio, vegetable grower John Hirzel recorded 25-percent yield increases in tomatoes that were started in the greenhouse in mixtures of one-third compost, then transplanted to the field into soils amended with 10 to 12 tons of compost per acre. Hirzel, who died in 2000, found that tomatoes grown with more compost have better resistance to bacterial canker, bacterial spot and bacterial speck,. 'As soon as they germinate, they are living in a soil that has natural bacteria and fungi,' he said in a 1999 interview.

On the other hand, farming practices that cause imbalances in nutrition or other factors can lower natural resistance. High nitrogen fertilizer levels can fuel the germination and growth of many weed species, boost the incidence of diseases such as Phytophthora, Fusarium and corky spot, and stimulate outbreaks of aphids, mites and other insects.

Some herbicides lower the resistance of crops to invading disease-causing organisms. Even more serious, as it decays, glyphosate-treated vegetation can create flushes of Fusarium, Rhizoctonia and other pathogenic fungi.

Rotation, in the absence of known pests, has improved growth and yields in many crops by about 10 percent. Longer rotations tend to increase crop yields more than shorter rotations; yields of corn and wheat grown as part of three-year rotations exceed those in two-year cycles or in continuous monocultures. Adding organic matter - through cover cropping, animal manures and crop residues - boosts crop performance and may improve pest tolerance.

For three decades, Dick Thompson has planted cover crops, managed weeds like covers instead of like pests, and lengthened and expanded his crop rotation. 'I'm not saying we don't have any insect problems, but they do not constitute a crisis,' Thompson says. 'We don't have to treat for them. We haven't done that for years.'

A word of caution regarding no-till. If not managed properly, eliminating tillage can provoke problems. Annual weed populations can build more rapidly if seeds stay on the soil surface, soils may warm up more slowly in the spring and, under some conditions, no-till may increase plant disease because some pathogens survive better in undisturbed soils.

You may need to till to control perennial weeds that crop up in undisturbed fields. Soils - especially wet, poorly drained ones - may need tillage to alleviate compaction by heavy machinery. In such cases, devise a rotation that involves tillage only during selected years or seasons, or use strip-till or ridge-till instead of no-till. In regions with cool, wet springs, no-till may not work well for early planted crops. You may want to talk to professionals from the Extension Service or the Natural Resources Conservation Service before changing your tillage regime.

On his Boone, Iowa, farm, Thompson uses a corn- soybean-corn-oats-hay rotation, with at least four different kinds of hay. He shreds weeds in his ridge-till system, then cultivates and lays them between the rows, turning a pest into a mulch.

At 6 percent, his soil organic matter is now double that of neighboring conventional operations. 'You can tell it by working it,' Thompson says. 'I can do things with a cultivator that others can't do. I'm not moving big clods but fine soil. The dirt flows and allows you to cover up the weeds.'

Because his soil no longer blows or washes away as easily as it once did, Thompson's high ground is becoming more productive. 'Our yields are just as good - if not better - on the hills as they are on the low ground.'